Effect of seasonal changes in bottom water oxygenation on sediment N oxides and N2O cycling in the coastal upwelling regime off central Chile (36.5°S)

An intensive and seasonal coastal upwelling process, which attains maximal expression during late austral spring and summer, drives well-known changes in organic matter production and, therefore, in O2 content in the water column. These variables have a concomitant effect on N sediment processes over the continental shelf off central Chile (36.5°S), which, in turn, can affect the NO3-, NO2-, and N2O content in the bottom water. Hydrographic characteristics, benthic NO3- and NO2- fluxes, and denitrification rates were measured from 1998 to 2001 (with at least seasonal frequency). In order to elucidate how benthic N2O recycling responds to different O2 and nutrient levels and how it affects the bottom water N2O content, net N2O cycling was measured in December 2001 in sediment slurry incubations under different manipulated dissolved O2 levels (anoxic: 0 μM; hypoxic: 22.3 μM; oxic: 44.6 μM) and without (natural) and with the addition of NO3- and NH4+ (enriched experiments). Dissolved O2 and NO3- contents (and also PO43-) showed clear seasonal patterns according to the oceanographic regime, i.e., from hypoxic waters rich in nutrients during the upwelling season to oxic waters with less nutrient contents during the non-upwelling season. The bottom water, on the other hand, was influenced by benthic organic mineralization, which consumes O2 as well as other electron acceptor N-species such as NO3-. Benthic NO3- fluxes (2.62-5.08 mmol m−2 d−1) were always directed into the sediments, whereas denitrification rates varied from 0.6 to 2.9 mmol m−2 d−1. N2O was also consumed at rates of 5.53 and 4.56 μmol m−2 d−1 under anoxia and hypoxia, but N2O consumption rates were reduced to almost half under oxic conditions in both natural and a NH4+-enriched experiments. With the NO3--enriched experiments, however, N2O consumption was very high (up to 24.25 μmol m−2 d−1) under anoxic and hypoxic conditions, suggesting that high NO3- levels induce more N2O reduction to N2 by denitrification. N2O production rates were only measured when oxic conditions were observed in the NO3--enriched experiment, suggesting some role of nitrification. Thus, N cycling in the sediments seems to affect the observed NO3-, NO2−, and N2O content in the bottom water and, therefore, in the entire water column due to vertical advection associated with coastal upwelling.